1. Technical Field
The present invention relates to a piezoelectric resonator and in particular, to a piezoelectric resonator having a mesa type piezoelectric vibrating element in which the thickness of the vibrating portion is larger than that of the peripheral portion.
2. Related Art
A mesa type piezoelectric vibrating element is known as a form of a piezoelectric vibrating element which can keep vibration energy therein and which has high productivity, the form being based on a bevel or convex type piezoelectric vibrating element.
In the mesa type piezoelectric vibrating element which has a level difference on the boundary of the vibrating portion and the peripheral portion, however, spurious vibrations, such as flexure vibrations which are unnecessary vibrations, increase with the influence of the level difference. Under such a background, JP-A-2006-340023 discloses a technique of suppressing spurious vibration by optimizing the position of a stepped portion between a vibrating portion and a peripheral portion.
Moreover, JP-A-2008-263387 discloses a technique of suppressing spurious vibration and reducing the CI value by optimizing the size (depth) of a stepped portion in addition to the position of the stepped portion. Moreover, JP-A-2008-306594 discloses a technique of improving the effect of spurious vibration suppression by extending the formation position of an excitation electrode up to a peripheral portion with a small thickness and setting the end of the excitation electrode at the position of a valley of unnecessary vibration.
Thus, for a piezoelectric resonator which adopts a mesa type piezoelectric vibrating element, various techniques have been proposed which suppress flexure vibrations by optimizing the position of the end edge of a mesa portion, that is, a thick portion or the position of an end edge of an excitation electrode on the basis of the relationship with the displacement of flexure vibrations.
However, as disclosed in JP-A-2008-263387, the CI value can be decreased as the rate of digging quantity of the thick portion is increased, but there is no change in the CI value if the rate of digging quantity exceeds a predetermined range. Depending on the situation, a phenomenon occurs in which the CI value increases (deteriorates). In order to solve this phenomenon, JP-A-10-308645 discloses a technique capable of ensuring the digging quantity (increasing the thickness of a stepped portion) and reducing the CI value by forming a mesa portion with multiple steps.
As disclosed in JP-A-2006-340023, JP-A-2008-263387, JP-A-2008-306594, and JP-A-10-308645, the mesa type structure can suppress an unnecessary wave by specifying the position of the stepped portion. Moreover, the CI value of principal vibration can be decreased as the rate of digging quantity of the stepped portion is increased, but deterioration of the CI value, that is, the phenomenon that the CI value increases occurs if the rate of digging quantity exceeds a predetermined value. When manufacturing errors and the like are taken into consideration, it is difficult to practically increase the digging quantity up to the threshold value.
Moreover, in the configuration disclosed in JP-A-10-308645 in which a mesa portion is formed to have multiple steps in order to increase the relative digging quantity, the CI value may be reduced, but the photo process for forming the mesa portion with multiple steps is increased. As a result, there is concern regarding a situation where the productivity decreases markedly.